We have the Premium Family Plan and use Sonos at home. When we stream Spotify on our Sonos system, I am unable to stream to my phone or otherwise at the same time. I am the Plan Manager. I've even used a different email address for our Sonos system to try and alleviate this issue. Spotify also says I am a Plan member as well as my wife. I have personally logged her out of Spotify and into the account that we created for her, so I do not believe we are trying to use Spotify simultaneously using the same profile. For the life of me, I cannot figure it out. Please help before I take my own life!
We recommend that both you and your wife use thelog out everywhere feature and also unlink your Spotify account from the Sonos app as described here. Afterwards make sure to log in to your personal accounts and link them in the Sonos app on your devices. That should do the trick.
Hope this helps. Keep us posted on how it goes.
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“Music is a language that doesn’t speak in particular words. It speaks in emotions, and if it’s in the bones, it’s in the bones.” - Keith Richards